3 Cylinder Cars - Real World Fuel Consumption

[NHB]

Originally Posted by cRiPpLe_rOoStEr

The longer stroke-to-bore actually is one of the reasons that lead those 3-bangers to have a better low-end responsiveness. At least in my country they're still more restricted to entry-level subcompacts while forced induction is the exception, not the rule.

Why would an undersquare engine be any more capable to produce low end torque than an oversquare engine?

I checked what VW offers in Brasil. Their 3-cylinder engine is almost square, but VW's 4-cylinder 1.6 liter engine is much more undersquare.

[cr45]

Originally Posted by NHB

Why would an undersquare engine be any more capable to produce low end torque than an oversquare engine?

I checked what VW offers in Brasil. Their 3-cylinder engine is almost square, but VW's 4-cylinder 1.6 liter engine is much more undersquare.

The undersquare engine has a higher mean piston velocity - and peak piston velocity - for a given rpm compared to the oversquare engine. This results in gas velocity through the inlet valves becoming choked - reaching sonic velocity - at a lower rpm compared to the oversquare engine. This holds true even if the valve size's are the same but is more pronounced if the bigger bore uses larger valves.

Peak torque occurs when the flow becomes substantially choked during the inlet phase for any engine.

Another reason is that there is less heat loss via the piston crown and cylinder head for the undersquare engine due to the smaller surface area's of the piston crown and cylinder head. As the peak combustion temperatures occur just after top dead centre, this also aids the sfc of an engine.

A 3 cylinder 1.0 lit engine has less total piston crown and cylinder head surface area compared to a 4 cylinder 1.0 lit engine, for the same bore/stroke ratio. It also has a longer stroke. The net result is an engine with better low speed pulling power plus better fuel economy.

[cr45]

This equation can be used to calculate the ratio of piston crown and cylinder head surface area for one engine compared to another.

n - number of cylinders
V - engine capacity
e - bore to stroke ratio

Surface Area Ratio = (n2 / n1)^0.3333 + (V2 / V1)^0.6666 + (e2 / e1)^0.6666

If comparing 2 engines that have the same capacity and bore/stroke ratio then the equation simplifies to (n2 / n1)^0.3333

Using this equation to compare the piston crown surface area of a 4 cylinder vs a 3 cylinder engine we get

ratio = (4 / 3)^0.3333

ratio = 1.1006

Hence a 4 cylinder engine has 10% more piston crown and cylinder head surface area compared to a like sized 3 cylinder engine. This gives the 3 cylinder engine a nice head start in the fuel economy stakes.

[Ecky]

Just read about the Civic getting a 1.0 turbo in many parts of the world:

2017 Honda Civic 1.0 i-VTEC Turbo SR review review | Autocar

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

Just read about the Civic getting a 1.0 turbo in many parts of the world

It's quite surprising to see that Honda is going more serious about turbocharging nowadays, after a long time focusing on natural aspiration (except for some of their JDM-only Kei cars and some Euro-spec versions of the Civic and the Accord fitted with turbodiesels outsourced from GM and Rover).